Radio receiving apparatus



May 15, 1934. w PLACE 1,959,062

RADIO RECEIVING APPARATUS Filed Nov. 24, 1931 un 10' 'n l IIIIIIIPLIIIIIIIPLI'IIII JNVENTOR l'fillard p. pimp; QRW

HIS ATTORNEY Patented May 15 1934 v UNITED STATES RADIO RECEIVING APPARATUS Willard P. Place, Wilkinsburg, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application November 24, 1931, Serial No. 577,061 3 Claims. (Cl. 179-171) The present invention relates to the art of radio reception, and comprises a new method of automatically regulating the sensitivity of an audio frequency amplifier to maintain a substan- 5 tially constant output voltage level irrespective of changes in the level of the voltage impressed upon the amplifier; and includes also apparatus adapted to carry out the method.

The new method may be employed to cause rapid desensitization of an amplifier upon increase in input voltage level with slow recovery of sensitization upon a decrease in the input voltage, or if desired may be employed to give a rapid response upon a change in either direction of input voltage level; thus being suitable for use with amplifiers amplifying either voice and music, or signals.

Broadly the new method comprises the rectification of a portion of the alternating audio frequency output of an amplifier, and theutilization of this rectified portion to so vary the grid bias of the tube or tubes of the amplifier as to increase the negative bias thereof with increase in output voltage. 1

When the negative grid bias of a tube is small,

a change thereof makes but slight difference in the amplification, but, when the negative bias is large, the same change makes a decided difference in the amplification. Thus a very desirable characteristic of the new method, whereby the grid bias is varied directly as the output voltage, is that the sensitivity will be only slightly affected by the automatic volume control when the output voltage -is low, but becomes increasingly effective as the output increases.

For a better understanding of the invention reference may be had to the accompanying drawing of which Fig. 1 illustrates diagrammatically the new automatic control applied to the audio frequency stage of a radio receiver, and Fig. 2 is a fragmentary view illustrating diagrammatically an alternative arrangement for a portion of the control of Fig. 1 adapted to insure more uni form operation of the control over a wide range of voltage levels. i

In Fig. 1 three audio frequency amplifiers of a receiver are indicated at '1, 2 and 3 and an output tube at 4. Amplifiers l, 2 and 3 are shown as tubes of the screen grid type and tube 4 as a triode.

As. the type of amplifier illustrated is well known and forms no part of the present inven tion it will be suificient to describe only those parts of Fig. l which comprise the automatic con- 66 trol or which are modified by the introduction of the control. ,In accordance with the invention the output transformer 5 of the amplifier is provided with a third winding 6 connected across the A. C. terminals of a full wave metal oxide rectifier 7. The high potential D. C. output ter- 60 minal of the rectifier is grounded and connectedthrough a capacity 8 and resistor 9 in parallel to the low potential D. C. output terminal of the rectifier.

The end of grid leak resistor 10 for tube 1, re- 5 mote from the grid of this tube, instead of being connected directly to ground is connected to one plate of a condenser 11, the other plate of which is grounded; and to the low potential D. C. terminal of rectifier 7. Similarly grid leak resis- 70 tors 12 and 14 for tubes 2 and 3 respectively are connected at their ends remote from the grids of the respective tubes to plates of condensers 13 and 15 respectively, the other plate of each of which is grounded, and to the low potential D. C.

terminal of the rectifier. Thus capacities 8, 11, 13 and 15 and resistor 9 are all connected in parallel'in the output circuit of the rectifier.

Capacities 11, 13 and 15are not essential to the operation of the volume control but, as they prevent instability due to coupling through the leads between resistor 9 and grid leaks 10, 12 and 14, their use is recommended.

When an alternating voltage is impressed across the input terminals 16 of the amplifier, it is amplified by tubes 1, 2 and 3, and a resulting voltage is impressed across the transformer 5 by tube 4. The percentage of the output voltage impressed across the A. C. terminals of rectifier 7 is rectified and utilized to charge the capacities 8, 11, 13 and 15 up to the peak voltage across the rectifier. When the output voltage remains constant a balance is quickly established and the only power taken by the volume control is that substantially negligible amount necessary to make up for that which leaks away through resistor 12 and for that wasted in the rectifier. Upon an increase in output voltage, the negative charge upon capacities 8, 11, 13 and 15 will be correspondingly increased with consequent increase in negative grid bias of tubes 1, 2 and 3. The change in the amplification effected by this increase in negative grid bias will depend upon the magnitude of the output voltage change and upon the former negative bias of the amplifiers. 1 5 If the negative bias was already high the correction will be substantial, whereas if the tubes were operating with a small negative grid bias, a lesser change in amplification will be effected by the same change in output voltage. As there is nothing in the circuit to cause any appreciable delay the desensitizing correction, of whatever magnitude, will occur with considerable rapidity.

Upon a decrease in the input voltage, the output voltage is correspondingly decreased and the peak voltage applied across rectifier '7 is no longor equal to the voltage to which the capacities are charged. Consequently the charge on capacities 8, 11, 13 and 15 will gradually leak away through resistor 9 and through rectifier '7 in the high resistance direction until a balance is reestablished at a lower negative grid bias of the tubes. As the time required for the amplifier to regain its sensitivity depends upon the particular values of the capacities and of resistor 9 and upon the constants of rectifier 7, reguiation thereof may be readily effected by suitable choice of these elements. When voice or music is to be amplified, the constants of the circuit should be so chosen as to require an appreciable length of time for recovery of sensitivity for as the alternating voltage corresponding to speech ormusic varies widely in magnitude, the quality of reproduction would be poor if the automatic volume control should operate to increase the sensitivity with too great rapidity. With constants of the circuit so chosen. that a few cycles of high intensity would be sufficient to desensitize the amplifier while a considerable length of period of low intensity would be required to regain sensitivity the automatic volume control would most nearly simulate the ideal condition wherein a predeterminedsensitivity is maintained until the input voltage level changes. Moreover the dif- "ference in the rates of correction, thus obtained For this purpose resistance 9 or the capacity of the volume control circuit or both should be aptecting the output tube, of the amplifier from overloading when operating close to its capacity. However, where uniform speed of operation of the volume control at all voltage levels is desired or where the input voltage normally varies over a considerable range and protection of the I output tube against overloading is not considered essential, the resistor 9 of Fig. 1 may be replaced by some suitable form of constant current de- ;vice. For example, as shown in Fig. 2, a vacuum tube 9a having its anode connected to the grounded high potential D. C. terminal of the rectifier and its cathode connected 'to the low potential I). C. terminal of the rectifier, may be substituted for the resistor 9. With such an arrangement, a-more uniform rate of discharge of the capacity of the circuit at all voltage levels may be obtained by adjustment of the temperature of the cathode of tube 9a, as by resistor 17,

to insure that a desired current will now when th capacities of the circuit are to be discharged; the magnitude of the current depending more upon the rate at which electrons are discharged fromthe cathode than upon the potential difference across the tube.

The new method of volume control and the new apparatus for practicing the method have now been described-in connection with the drawing. It will be apparent that the'new method comprises the automatic adjustmentof the negative grid bias of the tubes of an audio frequency amplifier in response to changes in direct current voltage of a rectified portion of the output voltage; the adjustment being such,.preferably, as to cause more rapid desensitization of the amplifier with increase in output voltage than recovery of sensitivity with decrease in output voltage. The apparatus for practicing this method permits of a wide range of adjustment of the rates of response at one or any voltage level and may be arranged to insure. substan-. tially uniform rate of response at all voltage levels, or to cause more rapid desensitization at high voltage levels as desired.

The following is claimed: jlilO 1. The method of regulating an audio frequency amplifier to maintain a substantially constant output voltage level which includes rectify-r ing a portion of the output voltage and so adjusting the negative grid bias of the various tubes 05 of the amplifier in response to changes in said rectified voltage as to relatively rapidly desensitize the amplifier with increase in said rectifled output voltage and relatively slowly increase the sensitivity with decrease in said rectified output voltage. a

2. The method according to claim 1 wherein given changes in output voltage when the negative grid bias of the tubes of the amplifier is large are arranged to effect a greater change in 115 the sensitivity of the amplifier than are changes in output voltage of the same magnitude when the negative grid bias of the tubes is small.

3. The method of regulating the amplification of an audio frequency amplifier which comprises 12ft) applying to the grid of each tube of the'amplifier.

a negative voltage which varies with the charge upon a capacity charged by a rectified portion of the output voltage to cause rapid increase of the negative grid bias of the tubes during charg- 12g ing of the capacity and relatively slow decrease of the negative grid bias of the tubes during discharge of the capacity.

4. The combination with an audio frequency amplifier having at least one amplifying tube leg of an automatic output voltage level control circuit comprising a full wave rectifier for rectifying a portion of the output voltage, a capacity connected across the direct current terminals of the rectifier having its high potential plate grounded 135. and its low potential plate so connectedto the grid ofsaid tube as to vary the negative bias thereof with the charge upon said condenser, and means for permitting a relatively slow discharge of said condenser when the output. volt age of said rectifier decreasesbeiow that to which said capacity is charged. 5. The combination according to claim 4 where! in said last-mentioned means comprises a resisto'r connected-across said capacity whereby the rate of discharge of said capacity is greater when the charge thereof is greater. 1

6. The combination according to claim 4 wherein said last-mentioned means is a device adapt- 1 ed to transmit substantially constant current m 50;

one direction when the potential thereacrossin one direction exceeds a predetermined valu 7. The combination with an audio Irequency amplifier having a plurality of amplifying tubes each provided .with a grid biasing resistor, of an automatic output voltage level control comprising a full wave rectiner for rectifying a portion of the output voltage, a plurality of capaci-.

ties each having one plate connected to one of said grid biasing resistors and to the low potential direct current terminal of the rectifier and each havingits other plate grounded. the high potential direct current terminal of said rectifier being also grounded, an additional capacity' connected across the direct current terminal of said-rectifier in parallel with said first- 9. The combination according to claim 7 wherein said last-mentioned means is a device adapted to transmit substantially constant current in one direction when the potential thereacross in one direction exceeds a predetermined value.

P. more. 

